Alkaline permanganate solutions are utilized for etching plastics material, for example in the manufacture of printed circuit boards but also prior to metallization of machined parts made of plastics for sanitary appliances, for the automotive industry, for precision mechanics industry or for furniture mountings. These solutions usually contain 30 to 100 g alkali permanganate and 30 to 100 g alkali hydroxide per 1 liter aqueous solution. The potassium permanganate being less expensive than sodium permanganate is normally used at a concentration below about 65 g/l and the considerably better soluble sodium permanganate is used at a higher concentration than 65 g/l. Typically potassium and sodium hydroxide are used as alkali hydroxide source. In general these solutions are operated at a temperature of from 60° C. to 98° C. Normally the plastics surfaces are treated with an organic swelling agent prior to the actual etching process in order to facilitate etching.
Due to the etching treatment of the plastics surfaces permanganate species are primarily reduced to manganate species, the manganate species on their part being disproportionated to permanganate species and to manganese dioxide. During this reaction considerable amounts of manganese dioxide sludge may be produced in the etching solution. A continuous operation of the method is not possible due to consumption of permanganate. Moreover the method is very expensive since permanganate salts must be added to the solution and waste material must be disposed of permanently.
In order to solve these problems a regenerating method has been developped. This method is described in EP 0 204 399 A1. In this method an oxidizing agent is added to the spent permanganate etching solution in an amount which is sufficient for reoxidizing manganate species in the etching solution with manganese compounds having an oxidation number being smaller than +VII thereby forming permanganate. Inorganic bromates, hypochlorites, chlorites, chlorates, peroxodisulfates, monopersulfates as well as the mixtures thereof are proposed as oxidizing agents. The oxidizing agents are reduced during the regenerating reaction and due to this reduction reaction are consumed.
This method has however proved to be complicated. Moreover additional substances are formed in the etching solution due to replenishment of the oxidizing agents. As a consequence the solution must be discarded. For this reason an electrochemical regenerating method has been developped, no undesirable reaction products being evolved due this method. Such a method ist described in EP 0 291 445 A2. This method is utilized to regenerate permanganate etching solutions serving to the roughening and cleaning of plastics surfaces, especially of printed circuit boards. The method consists of oxidizing manganese having an oxidation number of +VI to manganese having an oxidation number of +VII by electrochemical oxidation. This reaction is carried out in an electrochemical device and comprises an anodic reaction. For this purpose a cathode and an anode are arranged in an electrochemical device having a diaphragm preferably lying in between thereof. The anode and the cathode are connected to a current source in such a way that the cathode is polarized cathodically and the anode is polarized anodically. It has been explicitly pointed out in this document that optimum regeneration results are obtained with a device which is provided with two electrolyte compartments being separated from one another by a diaphragm. However, sufficient regeneration might also be achieved if a device would be used which is provided with one electrolyte compartment only, into which both electrodes are immersed.
Furthermore in EP 0 349 805 B1 a method is described for etching epoxy resin with an alkali permanganate etching solution with an etching rate of more than 3 μm, especially in bore holes in printed circuit boards, the etching solution containing 10 to 100 g/l alkali permanganate and at least 30 g/l alkali hydroxide. The method includes the following method steps: a. stabilizing the permanganate etching solution by electrochemical anodic oxidation at a direct current voltage of from 0.5 to 25 V and at a direct current density of from 0.1 to 20 A/cm2; b. adjusting the permanganate and OH— ion concentration by electrochemical and/or photometric measurement and appropriate additional dosage in the event of a deviation from the desired value.
A similar application for electrochemically regenerating chemical oxidizing agents that are used in galvanotechnics has been described by W. P. Innes, W. H. Taller and D. Tomasello in Plat. Surf. Finish., 1978, pages 36–40. This method relates to the regeneration of chromic acid etching solutions. In this case too the anode and the cathode of the regenerating device are separated from one another by a diaphragm, the diaphragm being a long porous ceramic cylinder.
A further application is disclosed in U.S. Pat. No. 3,470,044. The method described in this document relates to the eletrochemical regeneration of spent ammonium persulfate etching solutions. These solutions are inter alia utilized to dissolve metals, such as for example copper, cobalt, iron, nickel, zinc and the alloys thereof. For the regeneration reaction the etching solutions are passed through an anode compartment in an electrochemical cell, an anode being arranged in this compartment. This compartment is separated from a cathode compartment by a diaphragm this diaphragm being a cathode ion exchange membrane. A cathode is positioned in the cathode compartment. During operation of the electrochemical device persulfate is produced by oxidation of sulfate at the anode and metal ions are transferred to the cathode compartment the metal ions being reduced to elemental metal at the cathode.
A further device for regenerating permanganate etching solutions is disclosed in JP 6-306668 A. In this case the device comprises a regeneration container, the permanganate etching solution passing therethrough, further a plurality of cathodes and anodes being arranged in this container and being opposite to one another. The cathodes are partly covered by an electrically nonconducting layer, for example a polytetrafluor ethylene layer. For example the cathodes are designed as rods. The rods are only covered with the layer on certain sections. These sections alternate with other sections that are uncovered.
It has proven that the known methods for etching surfaces of plastics materials are complicated since either a large amount of chemicals is consumed for reoxidalizing manganese species having an oxidation number of less than +VII being formed due to the etching reaction or a large amount of energy is consumed. Especially it has turned out that a large amount of manganese dioxide sludge is formed in spite of continuous regeneration of the permanganate etching solution. This sludge must be isolated and discarded continuously. Moreover this sludge is blown up in the solution through the hydrogen evolved at the cathode so that this sludge possibly reaches the treatment container used for etching the plastics parts. Furthermore it has been observed after a long time of regenerating operation and connected herewith after a considerable sludge production that roughening of the plastics surfaces becomes uneven so that the degree of roughening of the plastics surfaces fluctuates.
Furthermore it has been observed, especially in cases in which large plastics surfaces per unit time are etched, that the efficiency of regeneration of permanganate gradually decreases. This behavior especially occurs when the SBU technique (sequential build up) is utilized. This technique involves a process in which individual circuit planes with plastics layers lying in between are manufactured sequentially. Each dielectric layer in circuit carriers must be cleaned and roughened with a permanganate etching solution in order to assure good adherence of the subsequent circuit line plane on this dielectric layer. Consumption of permanganate compounds and formation of the respective degradation products, for example of manganate and manganese dioxide, are considerable per unit time since large areas are to be treated with this process. Up to now a plurality of regenerating devices have been arranged in parallel in manufacturing plants in order to keep the concentration of permanganate in these solutions at the desired level at all. These devices require large floor space so that problems arise when the individual units are to be arranged side by side (treatment container and regenerating devices). These problems cannot be easily solved.
The problem of the present invention therefore comprises avoiding the disadvantages of known devices and methods and especially comprises finding a device being suitable for regeneration of permanganate etching solutions with high efficiency.